Gas laser with improved cathode life

ABSTRACT

There is disclosed a gaseous laser, whose cathode is a conductive coating upon the walls of a cathode volume, the geometric design of the cathode, cathode volume, and cathode-connecting volume being such as to impede sputtering of the cathode.

This invention relates to gas lasers. More particularly, it relates tospecial geometric configurations of gas lasers (especially laserenvelopes) which are inexpensive to manufacture and assemble. Thisinvention also relates to methods of manufacture, assembly, afabrication, and processing of such laser devices, including lasersystems incorporating such devices.

Gas lasers are widely known in the prior art. Reference is made to U.S.Pat. No. 3,628,175, issued to J. D. Rigden and U.S. Pat. No 3,149,290,issued to W. R. Bennett, Jr. and Ali Javan. See also Douglas C. Sinclairand W. Earl Bell, Gas Laser Technology, Holt, Rinehart and Wilson, c.1969.

Typically, a gas laser comprises an elongated hollow body containing asuitable gas and two laser mirrors positioned at opposing ends of thebody. Pump energy is appropriately introduced into the gas, causing apopulation inversion to exist therein. The existence of this inversionbetween laser mirrors of adequate reflectivity causes laser oscillationto develop in the gas. Alternatively, where the laser is used as anamplifier, the gas-filled body may simply be terminated by windowstransparent to the wavelength to be amplified.

Many different sources of pump energy have been shown to be suitable forthe initiation of laser action in gas lasers and are well known to thoseskilled in the art.

One important method of providing pump energy is to initiate an a.c.(alternating current) or d.c. (direct current) gas discharge in thelaser gas. Another method is to illuminate the laser gas volume withelectromagnetic radiation of a suitable wavelength, such as radiofrequency waves, visible light, or gamma radiation. Yet another methodis to initiate a suitable chemical reaction in the laser gas. Additionalpumping methods are known to those skilled in the art. Most of theembodiments set forth in this disclosure will refer specifically to agas laser embodiment in which a direct current electrical discharge ismaintained in the laser gas. However, the component configurations andmanufacturing methods described throughout this specification can besuitably used with a d.c. discharge in combination with any of a widevariety of pumping techniques such as radio-frequency pumping, and theuse of such alternative techniques is contemplated in the practice ofthis invention.

A wide variety of gases may be used in gas laser construction. Forexample, successful gas lasers have been made with the gaseous forms ofat least the following materials, separately or in mixture combination:helium, neon, argon, krypton, xenon, oxygen, mercury, cadmium, carbondioxide, carbon monoxide, water, sodium, potassium, cesium, copper,gold, calcium, strontium, thallium, carbon, silicon, tin, lead,nitrogen, sulfur, tellurium, florine, chlorine, iodine, manganese, zinc,boron, indium, germaenium, phosphorus, arsenic, antimony, bismuth,selenium, CN, HBr, DBr, HCl, DCl, HF, DF, HD, D₂, H₂, NO, CS₂, HCN, DCN,D₂ O, H₂ S, N₂ O, OCS, SO₂, CH₃ F, CH₃ OH, H₂ C:CHCl, and NH₃.

One preferred gas mixture for certain applications is helium - neon, thetypical composition being about 75 to 95 percent atoms of helium and 25to 5 percent atoms of neon. One specifically contemplated composition is87.5% atoms of helium and 12.5% atoms of neon, at a pressure of about 3Torr.

The laser mirrors define the ends of the optically active gas volume.Laser mirrors are generally required to have very precise surfacefigures, typically perfect to within about 0.1 wavelength of visiblelight. Multiple dielectric coatings are typically provided on mirrorsurfaces, so as to provide very high reflectivity, generally 98.5% to99.9% for helium-neon lasers, while permitting a small fraction ofincident radiation to be transmitted with little loss. These dielectriccoatings comprise a variety of materials such as silicon oxide, andtitanium oxide. The technology of laser mirror production is widelyknown, and such mirrors are commercially available from manymanufacturers, such as Spectra-Physics, of Mountain View, Calif. It iscontemplated in the practice of this invention to utilize any suitablelaser mirrors, with such parameters as reflectivity and radius ofcurvature to be determined by the nature of the laser gas and by therequirements of the specific laser use and application.

The laser mirrors must generally be precisely aligned with respect toone another and with respect to the axis of the laser bore. Typically, aperpendicular to the mirror surface at the laser bore axis must bealigned within a fraction of an arc minute of that axis. Techniques ofmounting and aligning mirrors to this tolerance are well known in theart. Although there may be particular mirror mounting and alignmenttechniques which are unusually well-adapted to the class of laserconstructions described herein, it will generally be possible to adaptto this present class of lasers any mounting or alignment technique thathas been used with prior-art lasers.

The ends of the volume filled with laser gas may be terminated directlyby the laser mirrors (the so-called "internal-mirror" laser), or may beterminated by vacuum-sealed optical windows, generally mounted atBrewster's angle, with the mirrors located outside the windows (the"external-mirror" laser). It is also possible to construct a laser inwhich one end is mirror-terminated and the other is window-terminated.

Although most of the examples given in this disclosure are stated interms of just one of these three forms, it should be understoodthroughout that in almost every case any of the three would be possibleand potentially useful variant.

An important element of the construction of a gas laser tube is themethod and materials used to seal or attach, to the glass or metalstructure of the tube, the mirrors or Brewster windows which define theends of the optically active discharge volume. This is typically done bymeans of epoxy resins. The critical requirements for the seal are:

1. It should be impervious to contaminating material such as water vaporfrom the outside environment;

2. It should be suitable to the processing environment, such as hightemperature;

3. It should not emit a significant density of contaminating materials;

4. It should constitute a seal of adequate mechanical durability.

A material commonly used to attach laser mirrors is Varian Torr-Seal®epoxy resin, which is applied as a thin bead around the edge of the partto be sealed. Other sealing methods and materials are known to thoseskilled in the art. Although certain of those methods may beparticularly suitable to the class of lasers contemplated in thisinvention, most known mirror-sealing methods will be readily adaptableto work with these lasers.

The procedures used for manufacturing the laser device should beadequate to remove all significant impurities. These procedurestypically include evacuation, possibly at an elevated temperature, andthe operation of a series of cleaning gas discharges in repeatedfillings of gas.

Where d.c. gas-discharge lasers are to be constructed, a cathode and ananode will be required.

Cold cathodes are typically constructed out of aluminum, magnesium,zirconium, or alloys thereof. The cathode must be prepared so as toretard sputtering. The production of a thin sputter-resistant oxidelayer over the electrode may be accomplished by operating a discharge inoxygen between the cathode and an auxiliary anode as described, forexample, in Section 7-3 of Gas Laser Technology, by Douglas C. Sinclairand W. Earl Bell (Holt, Rinehart and Winston, Inc., 1969) which ishereby incorporated by reference. Various other sorts of cold cathodes,such as the multicarbonate cathodes used in neon signs, can also beused. It is also possible to employ hot cathodes, in which thermionicemission from the cathode is a significant portion of its totalemission. It is contemplated, in one embodiment of the invention herein,to operate conductive-coating cathodes of the type described as eitherdischarge-heated or external-powersource-heated hot cathodes.

Cold cathode configurations are typically hollow. The hollowconfiguration is used because it tends to retard the ill effects ofsputtering and resulting gas clean up, and because it produces a compactstructure.

The anode may be of any suitable conductive material which can withstandthe cleaning procedures normally used in high vacuum technology.

It is common practice, in the prior-art technology specific tohelium-neon laser manufacture, to manufacture the laser cathode from apiece of aluminum tubing. There are several consequential costs in lasermanufacture using such a cathode. First of all, commonly availablealuminum tubing has been made by an extrusion process which leaves thesurface layer fouled with materials such as lubricating oil. Steps arerequired to remove the impure surface. One example of a suitableprocedure for removing the surface layer is machining of a fresh surfaceusing water as a lubricant instead of cutting oil, as disclosed by U.Hochuli, et al., "Cold Cathods for He-Ne Gas Lasers", IEEE J. QuantumElectronics QE-3, 612-614 (Nov. 1967), which is hereby incorporated byreference. Also, U.S. Pat. No. 3,614,642, which is incorporated byreference.

A second cost of using an aluminum tubular cathode is that somewhatcumbersome means must be provided to support mechanically the cathodeand to connect it electrically to a power supply outside the laserenvelope. For example, in U.S. Pat. No. 3,739,297, the electricalconnection and mechanical support means include a pin which must beheat-sealed through a glass section, with spring clips, welded wires,and other connecting means providing electrical connections between thepin and the cathode.

Many of the tubular glass laser device shapes which are commonly made bymanual flame working would be quite expensive or impossible tomanufacture with conventional high-volume glass-working machinery. Anexample of such a shape is the common "side-arm" laser construction, inwhich cathode and anode are placed in extension tubes or bulbs joined atthe side of the laser capillary. A sidearm laser construction of thiscommon sort is illustrated in FIG. 1-1 of Gas Laser Technology, supra.

Even the co-axial shapes which are more readily assembled on high-volumemachinery (and which also have the advantage to users of being compactin construction) will tend to be somewhat costly because of the need toproduce several vacuum-tight seals with relatively low strain and therequirement of assembling three, four or more tubular components withexcellent coaxial alignment. Also the laser capillary tube must retain ahigh degree of straightness during all the heating and coolingoperations which are implicit in manufacture.

In another known method of laser construction, a laser bore and channelsconnecting thereto are drilled into a block of fused quartz or otherinsulating material and separately-manufactured electrode envelopes arejoined to the connecting channels. Such as laser is illustrated in H. G.van Bueren, et al, "A small and stable continuous gas laser", PhysicsLetters 2,340- 341 (1 Nov. 1962) which is hereby incorporated byreference. The expense of drilling holes in glass materials may beuneconomical, especially when the depth of each hole must be long inrelation to its diameter. Likewise, large scale production may not befeasible.

Another method of laser envelope construction comprises the so-called"pressed" or "flat" glass laser as disclosed in our copending U.S.patent application Ser. No. 523,609 filed Nov. 13, 1974, herebyincorporated by reference.

The envelope of a gas laser is normally constructed out of glass such asa borosilicate, e.g., Kimble KG-33. However, many other glasses may beused so long as the thermal expansion coefficient is suitable for normalmanufacturing procedures and so long as the glass material itself doesnot contribute undesirable amounts of impurities to the gas discharge.

Other materials, such as metals, plastics, ceramics, glass-ceramics, andso forth may also be utilized. Plastics and other materials having highvapor pressures have generally been used only in flowing-gas lasers,which tend to be less sensitive to impurities than are sealed-off lasersbecause the constant gas replenishment in a flowing-gas laser reducesimpurity levels.

Throughout this disclosure frequent reference is made to the use ofglass as the basic material. It is important to understand that themethods and device configurations described are in almost every caseadaptable for use with machined or pressed ceramics, molded plastic, orany other insulating material, with appropriate alterations in choicesof sealants, thermal processings, and the like. The nature of thenecessary modifications will be apparent to those skilled in the art.

A common method of laser envelope construction has been theflame-working of tubular glass components. An example of such a laser isthe "single bore tube gas laser" described in U.S. Pat. No. 3,739,297,hereby incorporated by reference. Tubular glass has the advantage ofbeing a relatively inexpensive material, of being conveniently workedinto a variety of configurations, and of being a relatively convenientmaterial for the formation of gas-tight seals. One disadvantage oftubular glass structures is that such devices are more expensive thanmight be hoped when they are manufactured in large volume.

In accordance with the "pressed" glass embodiment, referred to above,there is provided a gaseous laser device comprising an envelope, acathode, a lasing gaseous volume, and an anode, the envelope beingdefined by at least two opposing substrates bonded together, thecathode, lasing gaseous volume, and anode being positioned withindifferent cavities of at least one substrate such that the cathode,lasing gaseous volume, and anode are commonly confined within theenvelope in an integrally connecting relationship.

Most laser applications have involved the use of relatively smallnumbers of lasers. However, there have been recent advances in the artcalling for relatively large numbers of lasers. One example of such anapplication is the video long-play record, or VLP, which is likely torequire a low-power helium-neon laser attached to a large fraction ofall television sets sold. Another such application is the laser-equippedpoint-of-sale scanner; which provides for automatic reading, by ascanned laser beam, of identifying tags on supermarket merchandise.

With these high-volume applications actually imminent, there comes to bea premium on the development of high-volume, low-cost long-lived lasersincluding manufacturing techniques.

It is widely believed that a major cause of failure in the gaseouslasers and decrease in effective life span is sputtering of the cathodematerial.

In accordance with the practice of this invention, such cathodesputtering is significantly reduced so as to extend the working life ofthe gaseous laser. More particularly, in accordance with this invention,the current density at the working surface of the cathode is decreasedsufficiently so as to substantially retard the deleterious effects ofsputtering and increase the laser life.

It is known in the prior art that a decreased current density willdecrease cathode sputtering. See, for example, John P. Goldsborough,"Design of Gas Lasers", in F. T. Arechi and E. O. Schulz-Dubois, eds.,Laser Handbook, Vol. I, North-Holland Publishing Co., 1972, at page 614.Goldsborough recommends a current density under 100 μA/cm² at 3 Torr,although other values have been used.

Furthermore, it has been found in a number of reported experiments thatthe rate of cathode material removal by sputtering in a glow dischargeis proportional to a high power of current density. See, for example,pages 138-144 of G. F. Weston, Cold Cathode Glow Discharge Tubes, LondonILIFFE Books Ltd, 1968, in which experiments are summarized showingsputtering rates proportional to the 2.5 or 3 power of current density.This text is hereby incorporated by reference.

In the practice of this invention, it has been discovered that thegeometry of the cathode surface and/or cathode volume, and the geometryof the "cathode connecting volume" which connects the laser bore to thecathode volume, can be specifically designed so as to maintain thecathode current density at a level low enough to prevent excessivesputtering. Specific geometric designs are disclosed and definedhereinafter.

It is desirable, in order to hold cathode sputtering and gas cleanup toan acceptable level in helium-neon or other gas lasers, to keep thecurrent density at the cathode surface relatively low, typically about50 to 500 microamperes per square centimeter.

In the practice of this invention, we have discovered and discloseherein a variety of design features whose function is to maintain thecurrent density, at every point on the cathode working surface, at alevel low enough to prevent excessive sputtering.

In the specific practice of this invention, we have observed that if agiven amount of current is to be distributed over a cathode of givensurface area with maximal uniformity (so as to minimize the risk ofrapid cathode erosion at points of high current density), then as muchas possible of the surface area of the cathode should be close to thepoint of discharge entry,

To state this design criterion mathematically, the surface integral Ishould be minimized, where, given a certain total surface area S= ds, wedefine

    = dds

where d is the distance from a surface element ds to the point ofdischarge entry. The center-entry position illustrated in FIG. 5 clearlymeets this criterion better than does the end-entry position of FIG. 1.

In setting forth our invention, it will be useful at times to describethe gas laser not in terms of the solid physical components which makeit up, but rather in terms of the volumes of space, which may beoccupied by gas, vacuum, or inserted objects, that are bounded by thesurfaces of the physical components of the laser envelope. There areusually several connected constituent volumes in a gas laser, each ofwhich has one or more well-defined functions. For example, a common d.c.glow discharge laser envelope, such as that depicted in FIG. 1-1of GasLaser Technology, may be thought of as having five constituent volumes;a cathode volume, whose function is to contain the cathode electrode andthe portions of the glow discharge which attach to the cathode (e.g.,the negative glow and the Faraday dark space), a cathode-connectingvolume whose function is to conduct the discharge from the cathodevolume to the laser bore, a laser bore (defined in this case by theinner surface of a piece of capillary tubing) whose function is tocontain the positive column in which laser action occurs, an anodeconnecting volume, and an anode volume to contain the anode electrodeand the discharge segments attached thereto (e.g., the anode fall if oneis present).

Other types of gas lasers may have different constituent volumes, but inmost cases it will be true that several recognizably different spatialvolumes enter into the laser construction, each of these volumes havingparticular operational functions.

It should be understood that by constituent volumes, we do notnecessarily mean spaces all of whose boundaries are defined by solidwalls. For example, in the device shown in FIG. 1-1 of Gas LaserTechnology, supra, the cathode volume is open at the point where thecathode connecting tube joins the cathode bulb. Nevertheless, it isclear to one skilled in the art that the space enclosed by the cathodebulb (and closed by an imaginary plane across the end of that bulb wherethe connecting tube enters) is a recognizably separate region of thelaser device, having well-defined functions. It is not always a matterof universal agreement how best to conceptually divide a given laserdevice into constituent volumes. For example, for some purposes it mightbe convenient to consider the cathode volume and the cathode-connectingvolume as a single unit. Nevertheless, it will be understood by thoseskilled in the art that most gas lasers have severalrecognizably-distinct constituent volumes.

Although most of the embodiments herein are stated in terms ofpositive-column glow-discharge, helium-neon lasers, which typically havecathode volumes, laser bore volumes, anode volumes, and one or moreconnecting volumes, many other varieties of lasers are contemplated tobe within the scope of the invention, such lasers requiring otherconstituent volumes well known to those skilled in the art. For example,metal vapor lasers may include metal-storage and condensation volumes.Lasers unusually subject to gas cleanup may include gas reservoirvolumes. Lasers subject to serious cataphoretic effects may includereturn path volumes (analogous to that illustrated in the U.S. Pat. No.3,628,176). Getter-containing volumes might be incorporated in manydifferent varieties of gas lasers, in order to increase resistance tocontamination during long-term operation, or to reduce the purityrequirements placed on the gas-filling station used in lasermanufacture. Coolant-flow volumes may be provided in lasers whichrequire the removal of excess heat.

Volumes may be provided for the insertion of optical elements, such asprisms, Brewster windows, intensity or phase modulators, gratings,aperatures, lenses, detectors, etalons, beam splitters, or other mirrorsadditional to the two normally required in the operation of a laser.

When we refer to the envelope of a laser, we mean the gas-tight wallwhich defines the periphery of the constituent volumes of the laser.However, in some embodiments, the envelope is considered not to includethe mirrors or windows which define the ends of the laser bore, theseend terminations having to be bonded to the envelope to complete a trulygas-tight structure. Likewise, electrical feed-throughs which conductelectric current from the inside to the outside of the envelope aregenerally considered not to be a part of the envelope, but rather to beseparate components which pass through the envelope in gas-tightfashion. In one embodiment of this invention, the envelope consists ofat least two opposing electrically-insulating components which aresealed, fused, or otherwise bonded together to form a structure which,with the addition of sealed-on mirrors, becomes gas-tight.

In this invention, several basic structural embodiments, each of whichis particularly suitable to high-volume manufacture, are contemplated.In one embodiment, one or more constituent volumes of the laser areincised, pressed, molded, machined, ground, etched, or otherwise definedin a single surface of a substrate. These volumes are then closed bysealing a second, flat, substrate to the incised substrate.

In a further embodiment, all or a plurality of constituent volumes ofthe laser are defined by the combination of depressions in the matingfaces of two component substrates, neither of the mating surfaces beingentirely flat. In this variation, it may be that neither of thesubstrates has any planar surface.

Another embodiment may be described as the "multiple-plate laser". Topand bottom surfaces of all or a plurality of this laser's constituentvolumes are defined by essentially-planar surfaces of top and bottomplates. The side surfaces of these volumes are defined by one or amultiplicity of plates essentially of equal thickness, which are sealedbetween the top and bottom plates.

Many obvious improvements to these three approaches are possible, andmore than one of them may be employed in a single device. Some of thepossible variations will be suggested in this disclosure, and otherswill be apparent to those skilled in the art.

Reference is made to FIGS. 1 to 12 illustrating some of the bestembodiments contemplated by the inventors in the practice of thisinvention.

In FIG. 1, there is illustrated an exploded, perspective view of atwo-part, "pressed" or "flat" laser construction which may be used inthe practice of this invention.

FIG. 2 is a cross-sectional view of FIG. 1.

FIG. 3 is an exploded, perspective view of a three-part laser preparedin accordance with this invention.

FIG. 4 is a perspective view of a cathode structure embodiment.

FIG. 5 is a plan view of a laser substrate having a length of tubing forinsertion of the discharge into the cathode volume.

FIG. 6 is a cross section of the cathode region of a laser incorporatingthe substrate and insertion tube of FIG. 5.

FIG. 7 is a schematic side view of a segmented tubular cathode for alaser, with an offset plot of current density versus the distance of thedischarge from its point of entrance into the cathode.

FIG. 8 is an exploded view of a laser envelope. It is similar to FIG. 1and incorporates the tapering connecting channel as required by oneembodiment of this invention.

FIG. 9 is a cross-sectional view of the anode region of the FIG. 8device.

FIG. 10 is a cross-sectional view of the cathode region of the FIG. 8device.

FIG. 11 is a different cross-sectional view of the cathode region of theFIG. 8 device.

FIG. 12 is a cross-sectional view of a modified cathode region similarto that illustrated in FIG. 11.

In FIG. 1, there is illustrated a substrate 1 and a cover plate 2. Thecover plate 2 contains a gas processing tubulation 3. Substrate 1contains a cathode volume 4, a connecting channel 5, a laser bore orgroove 6, and an anode channel 7.

Although the gas processing tubulation 3 is shown as connecting with thecathode volume 4, it will be understood by those skilled in the art thatthis gas processing tubulation may be conveniently positioned anywherein the device so as to introduce gas into the cathode volume, connectingchannel, and the laser bore.

Although not shown in FIG. 1, it will be understood by those skilled inthe art that a cathode will be introduced into the cathode volume area4. Likewise, an anode will be conveniently inserted and sealed,vacuumtight, into the anode channel 7.

In FIG. 2 there is illustrated a cross-sectional view of the entireassembled device of FIG. 1.

In FIG. 3 there is illustrated a modification of the embodiment ofFIG. 1. There is shown a top cover plate 32, a center plate 31, and abottom cover plate 31a. The top cover plate 32 contains a gas processingtubulation 33. The center plate 31 may be made up of one or moresegments. It comprises a cathode volume 34, a connecting channel 35, alaser bore 35, and an anode channel 37.

The cathodes and anodes are not illustrated in any of the FIGS. 1, 2 and3. However, it will be clearly understood by those skilled in the artthat cathodes and anodes of various geometric shapes may be convenientlyinserted into these devices. For example, a cathode would be insertedinto the cathode volume 34 in FIG. 3 and an anode would be inserted intothe anode channel 37 in FIG. 4. In actual practice, the cathodetypically may be in the shape of the walls of the cathode volume 34.

In FIG. 4 there is illustrated a cathode body 41 which would generallyconform to the wall shape of the cathode volume 4 in FIG. 1. The cathode41 contains a metal spade 42 which acts as a contacting tab forconnection to an outside source of electrical power. There is alsoillustrated an entrance hole 43 opening into a connecting channel suchas illustrated in FIGS. 1, 2 and 3.

One advantage of the set of geometries contemplated in one practice ofthis invention is that the internal surfaces of the contituent volumesof the laser are all laid open, which facilitates cleaning duringmanufacture, using a variety of cleaning processes such as plasmacleaning.

Another advantage of the laying-open of internal surfaces inherent inthis practice of the invention is that it facilitates coating by avariety of processes, such as vacuum evaporation, sputtering, chemicalvapor deposition, ion plating, and settling from solution.

One useful application of coatings is the manufacture of laser cathodesby placing an adherent coating of conductive material on the walls ofthe cathode volume of the laser device. An anode electrode may besimilarly manufactured.

It should, of course, be understood that the laser envelopes of thisinvention may be so constructed as to accept conventional electrodes,such as a metal pin anode or a hollow-tube cathode. A cathode of thisgeneral type is illustrated in FIG. 4. It is one important feature ofthis invention, however, that the laser envelopes described may easilybe constructed so as to accept coated electrodes, a feature not sharedby conventional tubular glass laser envelopes. To produce the cathodeand anode electrodes, for example, a vacuum evaporation system may beused to coat the upper and lower laser substrates, through a mask, withseveral hundred A of chrome (to act as an adhesion layer), followed byapproximately 10,000 A of aluminum. The mask restricts deposition to theregions which upon closure of the device would be the cathode and anodevolumes.

Any suitable metal or metalloid or semiconductor can be utilized as thecathode or anode material including the pure forms and conductive alloysof Mg, Be, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Tc, Re, Fe, Os, Rh, Ir,Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Hg, Al, Ga, In, Si, Ge, Sn, Pb, and soforth. Preferred materials include the pure or alloy forms of Al, Ti,Be, Mg, Ni, Ta, and Zr. For successful utilization of a particularmaterial, it may be necessary to protect the working surface with anappropriate barrier coating such as an oxide, nitride, etc. Suitablebarrier coatings include oxides or nitrides of Mg, Al, Si, Zr, Ti, Ta,and the rare earths.

It should also be noted that "thick-film" coatings may readily beapplied to the surfaces of laser components prepared in accordance withthe pressed glass and multiple-plate embodiments of this invention.These are typically coatings of conductive or insulating material whichare applied in slurry form, by such processes as screen printing,settling, or spraying, and then heat-processed to remove solvents andsolidify the material. Thick-film coatings have previously been employedin gas lasers. See, for example, K. G. Hernquist, "Low-radiation-noiseHe-Ne Laser", RCA Review, Sept. 1969, pp. 429ff, hereby incorporated byreference, in which a porous alumina lining, saturated with potassium,is used to form a conductive cathode surface.

The unique feature of a thick-film coating, as used in a pressed or flatdevice, is that the openness and accessibility of the structure wouldmake the application of coatings especially straightforward andeconomical.

An advantage of the open, easily coated structures is that passivatingcoatings may be easily applied to all interior surfaces. For example, itwould be possible to evaporate several thousand A of alumina or silicaonto all interior surfaces of the device shown in FIG. 1 prior to theapplication of metal layers, so as to prevent diffusion of impuritiesharmful to laser action out of the glass substrates. Suitable protectingmaterials include the oxides, nitrides, or carbides of B, Mg, Al, Si,Zr, Ti, Ta, and the rare earths.

The use of interior coatings on laser bores has been disclosed in theprior art, for example in U.S. Pat. No. 3,394,320 to G. K. Medicus. Whathas not been disclosed is the application of such coatings by methodssuch as electronbeam evaporation, which requires a clear line of sightfrom a source, or the application of a passivating coating to all orsome interior surfaces of a laser for the purpose of preventing emissionof harmful impurities from those surfaces.

The use of such passivating layers makes it possible to consider usingmaterials such as injection-moldable plastics to fabricate lasers, eventhough the relatively high vapor pressures of such materials might makethem unsuitable for use without such passivating layers except inparticular types of lasers such as flowing-gas lasers. If a passivatinglayer is constructed (perhaps in several sub-layers, including two ormore different materials) to have adequately low porosity under allconditions which the laser will experience, then the substrate materialwill not in any way interact with the gas discharge, and any substratematerial may be used whose thermal, mechanical, andelectrical-insulating properties are found suitable. In fact, thesubstrate material might be a conductor, such as invar or aluminum, solong as adequate protective insulating layers were provided at allpoints to avoid the shorting-out of the gas discharge by the substratematerial. An aluminum substrate heavily anodized on all surfaces mightthus be suitable.

Two particular advantages of using an organic thermoplastic material tomake laser envelopes are that:

1. Final alignment of the laser mirrors might be achieved by heating theplastic until it softened slightly, deforming the entire device or apart of it to obtain mirror alignment, and then permitting the plasticmaterial to cool. A comparable method of alignment has been used withglass laser envelopes, but the high softening temperature of glassesmakes this approach difficult with glass.

2. The several components of a thermoplastic laser envelope couldconveniently be heat-welded to produce gas-tight bonds, thus eliminatingthe need for a separate sealant material.

Another example of the use of interior coatings in a pressed laserenvelope is the production of conductive inserts in the laser bore toincrease laser power. See, for example, Yu. G. Zakharenko and V. E.Privalov, "Oscillations in the discharge gap of He-Ne laser and theireffect on the emission parameters", Opt. Spectrosc. 35, 434 ff (Oct.1973), hereby incorporated by reference. Zakharenko and Privalovdemonstrate that the use of metal rings, spaced within the positivecolumn of a He-Ne laser, can increase output power. The open structuremakes it very economical and straightforward to provide such inserts,either by coating the walls of the bore or by dropping in separate metaltubes before sealing on the top plate. If it is desired to provideelectrical contact to these rings, external connections could bemanufactured by techniques such as those disclosed hereinbefore foranode and cathode connections.

Yet another possible example of the utility of interior coatings arisesin the manufacture of high-current rare gas ion lasers. A commondifficulty in the manufacture of such lasers is that the laser bore israpidly eroded by the arc discharge. See, for example, William Bridges,"Materials and Techniques for Gas Lasers, Proc. 9th IEEE Conf. on TubeTechniques, 1968, pp. 117 ff, incorporated herein by reference. If thebore of such a laser is laid open, in a flat or pressed construction, itcould be straightforward to apply a very durable internal coating by aprocess such as flame spraying or plasma spraying. The substrates to becoated could be selected for such properties as high thermalconductivity, while the coating could be selected simply on the basis ofits providing maximal resistance to erosion by the arc.

Another advantage of the class of pressed or flat laser envelopescontemplated in one practice of this invention is that the flexibilityand precision of laser bore manufacture can be considerably enhanced.For example, in the conventional tubular gaseous laser, in which thelaser bore is a section of capillary tubing, it become relativelyexpensive to make bores much smaller than 0.040 inch in diameter, ormore precisely controlled in diameter than ±0.010 inch because of thedifficulties involved in accurate control of the drawing process. Incontrast, if the bore is manufactured by machining the surface of aglass plate, for example with a tungsten carbide or diamond grindingwheel of thickness slightly less than the desired bore width, it becomesstraightforward to make bores at least a factor of two smaller than0.040 inch in greatest dimension, and to control bore dimension to±0.001 inch or better.

One particular application of this advantage will lie in the manufactureof unusually short helium-neon lasers of relatively high power. Sincethe gain of these lasers is known to be inversely related to diameter,the availability of smaller diameters will make it possible toapproximately match the output powers of contemporary devices, but witha laser of shorter length.

A second application of the advantage will lie in precise control oftransverse lasing modes. It is common practice to restrict lasers todesired modes of operation, e.g. TEM₀₀, by controlling the size of theoptically limiting aperture in the laser cavity. In many cases, theaperture is the bore itself. The availability of economical methods forprecise control of bore diameter will make more economical the precisecontrol of bore diameter will make more economical the precise controlof transverse laser modes.

The necessity of the embodiments described herein for reducing cathodecurrent density became evident during experimentation with a laser madeaccording to FIG. 1. In one embodiment thereof, the cathode-connectingchannel 5 of the device in FIG. 1 has a width and depth of approximately0.15 at the cathode end. The cathode is of evaporated aluminum. When thedevice was operated for a period of hours, the cathode metallization wasobserved to erode seriously near the point where the connecting tube 5entered cathode volume 4. Observations of the relative brightness of thedischarge as a function of position in the cathode volume made itapparent that current density was highest near the entry point of thedischarge.

The device illustrated in FIGS. 5 and 6 represents one embodiment ofthis invention utilized to reduce cathode current density to a level lowenough to prevent serious cathode erosion.

There is illustrated in FIG. 5 a laser substrate 51, in which areseveral connected depressions: a cathode volume 54, a cathode-connectingvolume 55 comprised of connecting segments 55a and 55b, a laser bore 56,and an anode volume 57. Enlarged end sections 60 are shown on the laserbore 56, these enlarged end sections providing for reduced diffractionloss and for easier alignment of the laser mirrors (not shown) when theyare added to the device. The anode volume 57 is simply a small sideprojection on the laser bore. It is intended that the anode to be usedwith this particular device would be a thin foil 61 to be sealed vacuumtight between substrate 51 and a cover plate 52 (shown in FIG. 6). Theenlarged space 57 permits adequate electrical contact between the anode61 and the gas discharge, without any necessity for the anode toprotrude into laser bore 56 (which protrusion could interfere withproper optical performance of the laser).

A glass insertion tube 58, which may have a flared end section 59, issealed into the straight section 55b of the connecting channel 55, so asto conduct the gas discharge out into the heart of the cathode volume.

As can be seen in the cross-sectional view, FIG. 6, insertion tube 58may be bent, so as to release the discharge into the cathode volume atsome distance from the top wall of the cathode volume. It may also beseen in FIG. 6 that the top wall is formed by bonding a cover plate 52,similar to cover plate 2 in FIG. 1, to substrate 51.

Although a particular anode, anode volume, and cathode volume have beenillustrated in FIGS. 5 and 6, it will be obvious to those in the artthat other designs of these components may be used in combination withinsertion tube 58 which is the essential component of this embodiment.

In the operation of the device of FIGS. 5 and 6, the gas dischargespreads out as it travels from laser bore 56 through the taperedconnecting channel 55a. This channel 55a may alternatively be untapered.It then enters the cathode cavity through tube 58. The gas dischargeemerges from the end 59 of tube 58 (which may be flared to aid thespreading of the discharge) and must travel some distance through thecathode volume before it strikes a metal surface, which in this exampleis a conductive coating on the inside walls of volume 54. This distanceof travel permits the discharge current to spread considerably before itstrikes the metallization. The resulting low current density is suchthat serious cathode erosion is no longer observed, even after more than1800 hours of operation. Typical dimensions of one best embodiment of asuccessful device of the FIG. 5 type are as follows:

Cathode depth: about 0.7 inch

Cathode width: about 2 inch

Cathode length: about 6 inch

Depth and width of channel 55a:

0.060 inch at small end

0.25 inch at large end

Length of channel 55a: about 2 inches

Depth and width of channel 55b: about 0.25 inch

Length of channel 55b: about 0.5 inch

Length of tube 58: about 1.5 inch

Diameter of tube 58:

about 0.25 inch outside

about 0.15 inch inside

An important feature of the successful assembly of a device according toFIG. 5 is that the sealing of tube 58 into channel section 55b must besufficiently tight so as to prevent the gas discharge from establishinga "sneak path" beteen the outer wall of tube 58 and the wall of section55b; otherwise, the discharge could flow from cathode volume 54 toconnecting channel 55a without going through tube 58 at all. In theabsence of such a seal, cathode erosion may occur around the locationwhere tube 58 enters cathode volume 54. A suitable seal might beachieved by means of a heat-processed sealant such as solder glass, bymeans of a chemical bonding agent such as epoxy resin, by direct thermalfusion of tube 58 to channel 55b, or by any other suitable bondingmeans. It is also possible to achieve adequate resistance to asneak-path discharge by manufacturing tube 58 to fit its confining wallsvery closely, for example within 0.002 inch or better. In such anembodiment, no sealant is required, so long as adequate means isprovided to hold tube 58 in its proper position.

An important feature of the device illustrated in FIGS. 5 and 6 is thatthe side walls 62 of cathode volume 54 are sloped at about 45°. As aresult of this slope, an evaporated conductive coating applied to thewalls of the cathode cavity will be about 0.707 times as thick on thesidewalls as on the bottom 63 of the cavity. In contrast, the sidewallsof the cathode cavity shown in FIGS. 1 and 2 are in some locationswithin a few degrees of being vertical. Unless special evaporationfixtures are used, it is difficult to coat the near-vertical side wallsof cathode volume 4 in FIG. 1 to more than one-eighth of the thicknessof a coating on the bottom of the cathode volume.

The 45° slope thus makes it possible to coat all of interior surfaces toa thickness adequate to provide sufficient conductivity and resistanceto erosion by the discharge, without requiring the processing time tocoat some of the interior surfaces to an unnecessarily great thickness.

Another important feature of the device shown in FIGS. 5 and 6 is thatdischarge is brought into the cavity at a point near the cavity's centeralong its longest dimension. This is in contrast to the conventionalpoint of entry into an elongated laser cathode, i.e. at one end of thecathode, as illustrated in FIG. 3, and also in FIG. 1-1 of Gas LaserTechnology, Supra.

This side-entry design illustrated by FIG. 5 is one which would becomparatively more expensive to manufacture than the normal end-entrydesign, if conventional tubular laser technology were being used.However, with the integrated-manufacture techniques described in oneembodiment of this invention, it is straightforward and inexpensive toenter at the center of one side of the cathode.

The virtue of center entry may be understood by referring to FIG. 7.This figure is taken from a translated document prepared by the ForeignTechnology Division of the Air Force Systems Command. The document isavailable from the National Technical Information Service As AD 771885.The citation of the Russian original is O. A. Boyarchikov and A. S.Shipzlov, "Investigation of current distribution on the surface of ahollow cathode in a glow discharge in a mixture of helium and neon gas,"Trudy Moskovskovo Energeticheskovo Instituta: Radioelektronika, Nr. 108,1972, pp 89 -91. This is hereby incorporated by reference.

FIG. 7 shows schematically a segmentated hollow cathode 70 (comprised ofmultiple segments 71) for use with a helium-neon gas discharge, each ofwhich segments 71 can be separately connected to an ammeter. The datataken with the ammeter are represented in the graph plotted at thebottom of FIG. 7. The graph shows that current density drops off rapidlywith distance from the point (in this case the far right-hand end ofFIG. 7) at which the discharge enters the cathode.

In FIGS. 8, 9, 10 and 11, there is illustrated a laser which achievesthe purpose of keeping cathode current density to an acceptably lowlevel, by a different method than that employed in the device of FIGS. 5and 6. The tapered connecting tube 85, which is an essential feature ofthe embodiment of FIGS. 8 to 11, can be seen to be economical tomanufacture primarily because of the innovative techniques ofmanufacture contemplated in this invention.

In FIG. 8, there is illustrated a substrate 81 with a cover plate 82.The cover plate contains a gas processing tubulation 83, and an anodepin 88. Tubulation 83 and anode pin 88 are sealed vacuum-tight throughcover plate 82 by means of solder glass beads 89. Substrate 81 containsa cathode volume 84 having a conductive coating 84a over a portion ofits surface. An alternative embodiment could easily use a preformedinsert cathode, such as that illustrated in FIG. 4. That portion of overplate 82 which overlies and completes the boundary of volume 84 mayoptionally have a conductive coating 84b (as illustrated in FIG. 11). Itis understood that means are to be provided to electrically connectcoatings 84a and 84b to the negative terminal of a power supply. It isessential that the least dimension of volume 84 (i.e. the smallest oflength, width, and depth) be great enough to avoid undue concentrationof current in the vicinity of the discharge entrance. For commonhelium-neon lasers, this dimension should be at least about 1 cm.

Substrate 81 also contains a connecting channel 85, having thecharacteristic that it tapers in both depth and width so that at its endproximate the cathode volume 84, its depth and width are comparable tothe depth of cathode volume 84. This may be on the order of 2centimeters (cm). At its opposite end the connecting channel has depthand width substantially smaller, for example, between 1mm and 1 cm.Although the cross section of channel 85 is shown to be substantiallyrectangular, it may have any other convenient cross-section, such astrapezoidal or semicircular, so long as the ratio of depth to width isin the range between about 0.5 and about 2 at the channel's larger end.Considerably more difference between depth and width is permissible atthe smaller end of channel 85.

The limitation of the ratio of depth to width at the larger end of thechannel has been found helpful in avoiding noise in the laser output. Ifthe ratio of larger dimension to smaller dimension is permitted tobecome excessive, it has been found that the discharge will oftencontract to a diameter comparable to the smaller dimension, and that theposition of the discharge within the channel may then become unstable.The physical motion of the discharge within the channel then leads tolaser noise. On the other hand, by keeping the ratio of width to depthwithin the stated range, it has been found possible to ensure that thedischarge spreads smoothly to fill nearly the entire cross section ofthe channel, and that therefore no positional instability is possible.

The length of channel 85 must be great enough to permit the discharge tospread smoothly from a small diameter at one end of channel 85(proximate bore 86) to a larger diameter at the opposite end (proximatecathode volume 84). A typical length of channel 85 may be about 1 to 10cm, with a particularly preferred length being about 5 to 8 cm.

It will, in general, be desirable to keep channel 85 as short aspossible, consistent with the requirement of smooth spreading of thedischarge, since a shorter channel will tend to reduce the requiredstarting and operating voltages of the laser.

The innovative manufacturing processes contemplated in this inventionare clearly such as to permit many variations in the geometry of channel85. For example, although in FIGS. 8 and 11, the bottom of channel 85,at its end proximate cathode volume 84, is shown to be level with thebottom of channel 84, it is permissible for there to be a smalldifference in these levels, so that a step would occur of perhpas 1-2mmin passing from one to the other.

Although in FIG. 8 the path of channel 84 is shown to be a simple curve,it would also be permissible to have more complex paths, so long as thecross section of the channel monotonically increases from one end to theother.

A further variation is an increase of the cross section of channel 85 ina series of steps. The criterion of the maximum permissible step size isthat it must be small enough to cause no significant instability in thedischarge, since instabilities are in general a source of noise in thelaser output. The exact permissible step size will vary with gas mixtureand pressure, although with common helium-neon lasers it may be assumedto be less than about one centimeter.

Although FIG. 8 shows the tapered channel 85 entering cathode volume 84at one end, other configurations are obviously possible. In oneparticularly desirable embodiment, the tapered channel entersapproximately at the center of the long side of a generally rectangularcathode volume, the entry position being similar to that shown in FIG.5.

An important feature of the device illustrated in FIGS. 8, 9, 10 and 11is that those walls of the cathode volume 84 which are to be metallizedare tilted as far from the vertical as considerations of space makepractical. The unmetallized end wall of volume 84 is within a fewdegrees of vertical.

The edge of the metallization 84a, proximate the discharge entry, isgenerally curved concavely with respect to the discharge entry, so thatas the discharge emerges from tapered channel 85 it can tend to spreadout to fill the cross-section of volume 84 before striking the edge ofthe metallization. This design feature tends to reduce the density ofcurrent striking the metalization, thus prolonging device life.

Substrate 81 also contains laser bore 86, having enlarged end sections86a and 86b, the enlargement of these end sections providing for reduceddiffraction loss and for greater ease of alignment of the mirrors (notshown). It will be understood that these mirrors are added forcompletion of the device.

With respect to anode volume 87 in this particular device, the anodevolume is an indentation in cover plate 82, which may be provided, forexample, by grinding or pressing. Anode pin 88 projects into volume 87,the space around the pin in volume 87 being sufficient for the dischargeto have good electrical contact with pin 88, with no substantiallyadverse voltage drops or instabilities.

It is important to the optical performance of the laser, that unlessanode pin 88 is specifically intended to have a role in transverse modeselection, it does not protrude significantly into the cross-section oflaser bore 86, or at least that it does not protrude into that part ofthe cross-section in which substantial laser oscillation is occurring.

FIG. 12 illustrates another important technique for protection ofcathodes from excessive current density. FIG. 12 is a cross-section,similar to FIG. 11, of the cathode region of a device similar to that ofFIG. 8. There are illustrated substrate 121 and cover plate 122 formingthe boundaries of cathode volume 124 and connecting channel 125.Conductive coatings 124a, whose thickness is exaggerated for clarity,are applied to the walls of volume 124, and constitute the cathode.

It is well known in the art of manufacturing conventional sheet-metallaser cathodes that the edges of such cathodes tend to be particularlyvulnerable to erosion, because they are points of field concentration.Various stratagems are known for the protection of these edges. FIG. 12illustrates an innovative and elegant approach to edge protection, whichis particularly suitable for use with the innovative manufacturingmethods described herein. The figure shows an insulating layer 126,which may be any sputter-resistant material, such as silica, magnesia,alumina, or ytterbia, (or the barrier or protective coatings listedhereinbefore) which is applied to the edges of metallization 124a beforecover plate 122 and substrate 121 are assembled. The same wide varietyof coating techniques is available for producing layer 126 as forproducing the cathode itself, layer 124a.

In particular, layer 126 may be deposited by so-called "thin-film"techniques, such as sputtering or evaporation, or it my be deposited by"thick-film" techniques, as would be the case if layer 126 were producedby the screen-printing of a layer of solder-glass and heat-processing itto produce a continuous insulating layer.

Alternatively, 126 may represent a solid preformed insert of insulatingmaterial, which is placed in position during device assembly, andretained in position by the geometric design of cavity 124, by suitableadhesives, or by suitable mechanical clips.

Another technique for protection of cathode edges is illustrated inFIGS. 8, 10, 11, and 12. The metallizations 84a, 84b, and 124a arecarried up over the edges of the cathode cavities 84 and 124, so thatmuch of the perimeter of the metallization is sealed between the lasersubstrates, and therefore cannot be touched by the gas discharge. Thecavity edges over which the metallizations pass are assumed to besuitably rounded, so that electrical conductivity is maintained acrossthe edge, and so that the edge does not become a locus of intensifiedcurrent.

Yet another protection technique, which is particularly advantageouswhen practiced with the pressed-glass laser embodiment is to reducecathode current density by eliminating the usually-employed hollowcathode configuration. This technique may be practiced, for example, byconstructing a laser according to FIGS. 8, 9, 10, and 11, buteliminating either the upper metallization 84b or the lowermetallization 84a. The cathode volume would be correspondingly enlarged,so that the total metallized area, and hence the average currentdensity, would be unchanged.

The advantage of this technique may be understood by reference to FIG.7. It may be seen in FIG. 7 that in a conventional hollow cylindricalcathode the current tends to concentrate near the discharge entrance. Inparticular, the current density near the entrance may be expected to behigher than the so-called "normal" current density, j_(n), which wouldbe observed with an essentially-infinite planar cathode. This is thewell-known "hollow-cathode effect". See, for example, Weston, supra.,pp. 107-113. By switching to an essentially-planar cathode we mayeliminate this current-concentrating effect and enhance the uniformityof distribution of current.

In conventional tubular lasers, it would be cumbersome to provide anessentially-planar cathode with sufficient surface area, and withsuitable protection to prevent any of the edges of the cathode frombecoming points of current concentration. The edge-protection schemesillustrated in FIGS. 8 through 12 make it very straightforward andeconomical to protect the cathode edges, so that the combination of apressed-glass envelope with an essentially planar cathode is uniquelysuitable for maintaining cathode current density at a uniformly lowlevel.

It must be understood that the advantage gained by eliminating thehollow-cathode effect will to some extent be offset by the loss of onewell-known anti-sputtering effect of hollow cathodes, which is that suchmaterial as does sputter from the surface of a hollow cathode may easilybe redeposited elsewhere on the surface of the cathode. There may still,therefore, be particular combinations of current, pressure, and gasfilling, at which the hollow cathode can be shown to present a netadvantage in reducing sputtering.

We claim:
 1. In combination with a laser which includes a laser borecontaining a lasing gaseous medium, a cathode volume having wallsdefining the boundaries thereof and a metallization on the cathode wallsto serve as a cathode for said laser, and a connecting channel volumeconnecting said cathode volume to said laser bore, the improvement insaid cathode metallization wherein the edge of the metallizationproximate the discharge entry to said cathode volume is generally curvedconcavely with respect to the discharge entry, whereby the discharge asit emerges from said connecting channel volume spreads out to fill across-section greater than the cross-section of the cathode end of theconnecting channel volume before striking the edge of said metallizationto thereby reduce the density of current striking the metallization andthus prolong the life of the cathode.